The invention relates to an apparatus with which items can be formed of molten materials. Such materials predominantly concern glass or glass ceramics. Pressing is used as the forming method.
The pressing of glass has long been known as a forming variant. A piece of glass which is brought to a suitable temperature and has a sufficiently low viscosity at said temperature for deformation is usually brought between at least two tools which are then moved towards each other and enclose the gob in a space remaining between the two forming surfaces. The necessary force for deformation is applied to the tools. Usually, the force is maintained until the glass is sufficiently cooled in order to be removed from the tools without subsequent deformation.
During the pressing, the forming surfaces are in close contact with the glass to be deformed. As a result, even very fine structures on the forming tools appear on the newly formed surface of the glass part.
During the production of glass parts for optical purposes it is demanded, however, that the surface is free from any structures. Usually, such a surface on a component can only be achieved by grinding and polishing after the pressing. This usually entails high costs.
Class parts which cool off without contact to the forming tools show a so-called fire-polished surface without any damage. Since no forming surfaces form the glass during the solidification, it automatically assumes a shape which is determined by the surface tension which is virtually never the desired shape.
Tools are already known in which glass can be brought to the desired shape by a pressing process without damaging the fire-polished surface which Would be obtained during free cooling. The common aspect of these tools is that the forming surfaces consist of a permeable material and that prior to the introduction of the gob to be formed a fluid film is produced on the surfaces of the tool which prevents any contact between the gob and the tool surface and thus an damage to the surface of the gob. Said fluid film, and thus the contactless state, is usually also maintained during the pressing. Thus the Japanese published application JP 4-6114 describes a method in which the pressure rams are saturated with a liquid which evaporates during the pressing process and thus forms a glass film. This method comes with the disadvantage that the evaporation occurs in an uncontrolled way and an even glass film can thus only be produced with difficulty.
For the purposes of process security it is better to introduce the glass film in a direct and controllable way. For this purpose the forming surfaces are flowed through on the side averted from the glass with a gas or gas mixture and form a gas film between the tools and gob.
Press-forming of glass between forming surfaces covered with a gas film is described in the Japanese published application JP 2000-302473.
In pressing tools, the gas film introduced between the forming surface and glass part is compressed by the applied pressing force up to very small thicknesses of a few micrometers. In order to ensure that even through this very thin film no damage can be transmitted from the mould onto the glass part, the forming surfaces must have a very low roughness, so that no acute elevations over 5 μm are present on the surfaces. Moreover, the material on the forming surface facing the glass should neither impair the glass nor be attacked by the same.
Furthermore, in the case of short-term contact there should not be any damage to the forming tool.
Porous materials naturally show a certain roughness due to their pores. Since this roughness lies approximately in the magnitude of the pore size, it is necessary to use very fine-pored materials with pore sizes below 20 μm, preferably below 5 μm, for pressing tools. These materials lead to a high pressure loss for the gas flowing through the same.
If the flow resistance of the porous mould is high, it is necessary to work with relatively high supply gas pressures due to the thus caused pressure drop, which entails relatively high operating costs. There is furthermore the danger of a mould breakage because the pressure load acting on the mould is relatively high. On the other hand, it is known that a certain minimum differential pressure is the precondition for the even formation of the most stable possible gas film, which can be achieved in the most economical way by a mould base with relatively low values for gas permeability, but with the most even possible pore distribution over the surface.